Summary

Marine microbes play a key role in global nutrient cycling (1). Phytoplankton account for less than 1% of the photosynthetic biomass on Earth, yet contribute almost half of the world's primary production (2). In open oceans between ∼45°N and ∼40°S, cyanobacteria of the genus Prochlorococcus dominate the phytoplankton (3). Prochlorococcus is the smallest free-living phototroph; its genome is subject to miniaturization (4), and distinct ecotypes are adapted to the strong gradients of light and nutrients in the surface ocean (5). On page 183 of this issue, Biller et al. (6) identify a further striking feature of Prochlorococcus: the production of extracellular vesicles that may play a role in carbon cycling, gene transfer, and viral defense.